An electric-resistance-welded steel pipe or tube is typically produced by forming a steel strip into a pipe or tube shape and, using squeeze rolls, butt pressing and welding the transverse ends of the steel strip heated and molten by high-frequency current and facing each other. It is commonly known that the weld of the electric-resistance-welded steel pipe or tube is lower in mechanical properties such as toughness and strength than the steel strip before the welding.
The lower properties of the weld are caused by, for example, oxide-based weld defects called penetrators. The penetrators remain in the weld, and decrease the toughness and strength of the weld. To prevent the penetrators from remaining in the weld, a measure is typically employed to increase the amount of upset by the squeeze rolls so that oxidized melts formed during the welding are discharged to outside the pipe or tube.
A technique proposed to improve the properties of the electric-resistance-welded steel pipe or tube is an electric-resistance-welded clad steel pipe or tube. The electric-resistance-welded clad steel pipe or tube is an electric-resistance-welded steel pipe or tube produced using a clad steel strip that is obtained by cladding a steel strip as base metal with a metal strip (cladding metal) made of a material different from the base metal. Combining the different materials in this way makes it possible to obtain a steel pipe or tube having excellent properties by benefiting from the properties of both the base metal and the cladding metal. For example, in the case of using carbon steel as the base metal and stainless steel as the cladding metal, an electric-resistance-welded clad steel pipe or tube having both the corrosion resistance of stainless steel and the strength of carbon steel can be obtained.
However, if the amount of upset is increased in the case of producing the electric-resistance-welded clad steel pipe or tube using the clad steel strip as raw material, a phenomenon that the molten steel and the heat-affected zone (HAZ) of base metal 11 enter the welded seam part of cladding metal 12 (on the steel pipe or tube inner surface side in FIG. 7A) occurs, as illustrated in FIG. 7A. In particular, if the amount of upset is excessively large, the base metal 11 is exposed at the cladding metal-side surface of the steel pipe or tube (the steel pipe or tube inner surface in FIG. 7A). This impairs the performance as a clad steel pipe or tube exploiting the excellent properties of the cladding metal.
For example, in an electric-resistance-welded clad steel pipe or tube produced using, as raw material, a stainless clad steel strip having low carbon steel as the base metal 11 and stainless steel as the cladding metal 12 with a large amount of upset where the cladding metal forms the inner layer and the base metal forms the outer layer, the low carbon steel enters the seam part of the stainless steel, or the low carbon steel is exposed at the steel pipe or tube inner surface as illustrated in FIG. 7A. This causes a significant decrease in corrosion resistance near a welded seam part 14 at the steel pipe or tube inner surface. Such an electric-resistance-welded clad steel pipe or tube cannot deliver required performance, when used in an environment in which the steel pipe or tube inner surface is required to be resistant to corrosion.
The conventional techniques thus have difficulty in preventing, in an electric-resistance-welded clad steel pipe or tube as electric resistance welded, both of a decrease of the mechanical properties of the weld and impairment of the function as a clad steel pipe or tube. Known techniques for addressing such a problem subject the electric-resistance-welded clad steel pipe or tube to additional treatment.
JP S60-221173 A (PTL 1) discloses a method of producing a clad pipe or tube, whereby at least the cladding metal-side bead of the weld bead formed by butt welding the facing edges of a clad steel sheet or steel strip bent in a pipe or tube shape is removed by cutting to a depth reaching the base metal, and the cut portion is subjected to weld overlaying in which a welding material having the similar properties as the cladding metal is used.
JP S62-156087 A (PTL 2) discloses a method of producing a clad steel pipe or tube, whereby, after forming a clad steel strip into an open pipe or tube and electric resistance welding the joint edge parts, any of the following is performed: causing melting and solidification to a depth of the clad interface along the welded seam in which dissimilar metal has entered, to dilute the dissimilar metal; and weld-overlaying the seam part in which dissimilar metal has entered with the same type of metal as the cladding metal, and rolling the overlaid weld to dilute the dissimilar metal.
JP H5-154545 A (PTL 3) discloses a method of producing a welded clad steel pipe or tube having cladding metal on the inner surface side, the method comprising forming a blank sheet or blank coil of clad steel into a tubular body so that the cladding metal forms an inner surface, subjecting at least a portion of butted parts of the cladding metal to electric resistance welding, and then subjecting the non-welded butted parts to weld overlaying.